The invention relates to a process for the purification of gaseous effluents by photocatalysis. It also relates to the plant for the implementation of this process.
The reaction referred to as the photocatalytic reaction, also known as photocatalysis, consists in destroying the various organic and/or inorganic contaminants present in gaseous effluents and in particular the air by photochemical reaction, which is brought about by the irradiation of a catalyst under ultraviolet rays.
In essence, photocatalysis is initiated by activating a semi-conducting solid (photocatalyser, such as, for example, TiO2) with UV radiation at a wavelength of less than 380 nanometers, resulting in electronic changes within the semiconductor and leading, in the presence of air or water, to the creation of oxygen-comprising radicals at the surface of the semiconductor. These radicals attack the organic or inorganic compounds adsorbed on the semiconductor and, by sequence of chemical reactions involving the oxygen of the air, decompose the compounds until the final stage of the oxidation is reached.
The present invention relates more particularly, but non-limitingly, to the purification of the air. This is because the air is capable of comprising a large number of contaminants, including NOx, NH3, H2S, CO, O3, chlorinated or nonchlorinated C2-C4 alkenes, chloromethane, isooctane, benzene, toluene, xylene, isopropylbenzene, saturated aliphatic C1-C4 alcohols, methyl mercaptan, chlorophenol, nitrophenol, methyl tert-butyl ether, dimethoxymethane, C1-C4 aldehydes, acetone, formic acid, acetic acid, 2-methylpropanoic acid, dichloroacetyl chloride, dimethylformamide, trimethylamine, acetonitrile, pyridine, methanethiol or dimethyl disulphide.
As already said, use may in particular, but non-limitingly, be made, as photocatalysing agent capable of triggering the photocatalytic reaction, of anatase titanium dioxide TiO2, which, activated by UV light, is electronically modified so as to result in the formation of hydroxyl OH* and oxygen O* radicals capable of attacking the organic carbonaceous chains adsorbed on the TiO2 by decomposing them until the organic carbon is completely converted into carbon dioxide. However, it is possible to envisage other photocatalysing agents, such as those given in the group consisting of metal oxides, alkaline earth metal oxides, actinide metal oxides and rare earth metal oxides.
In practice, the photocatalysing agents are attached to substrates by means of bonding agents, in particular cellulose fibre or synthetic nonwoven substrates or glass fibre substrates. Such combinations, also denoted by the term xe2x80x9cfilter mediaxe2x80x9d, are widely known and disclosed, for example, in the international patent application of the Applicant PCT/FR 99/00748.
These filter media can be employed in plants for the purification of air by photocatalytic reaction, such as that disclosed in the document U.S. Pat. No. 5,790,934. The reactor forming the subject-matter of this document exhibits a plurality of fibrous substrates coated with a photocatalytic composition, which substrates, under UV radiation, are lapped by the gaseous effluent to be purified. It is also specified that the light source is substantially perpendicular to the fibrous substrate.
Even if this type of reactor is relatively simple in its design, the degree of purification of the gaseous effluent remains relatively low. Thus, from Example 1, it is observed that the degree of conversion of the formaldehyde is only 65% .
In other words, the problem which the invention intends to solve is that of developing a process for the purification of gaseous effluents which is more efficient than those provided in the prior art.
Another problem which the invention intends to solve is that of providing a plant for the implementation of the process which is simple in its design.
Another objective of the invention is to provide a plant which can be easily installed in areas where contaminated air is given off.
To achieve this, the invention provides a process for the purification of gaseous effluents by a photocatalytic reaction, according to which, under ultraviolet radiation in a single stage, the gaseous effluent is subjected to movements by which at least one first substrate is lapped and by which a second substrate is traversed, the said first substrate and the said second substrate being covered with at least one photocatalysing agent.
In other words, the process of the invention consists in combining lapping and traversing movement by the gaseous effluent to be treated of substrates coated with a photocatalysing agent. This is because it has been observed that such a combination under ultraviolet radiation makes it possible to greatly increase the efficiency of the purification with respect to the lapping or traversing movement alone by the gaseous effluent of a substrate covered with photocatalytic agent.
According to how the gaseous effluent is directed, the said effluent will lap the first and second substrates and will then traverse the said second substrate.
According to another embodiment, the effluent will only lap the first substrate and will traverse the second.
As already said, the invention also relates to the plant for the implementation of this process. Such a plant can be designated by the term of photoreactor.
According to a first characteristic, the plant for the purification of gaseous effluents by a photocatalytic reaction comprises:
a source of ultraviolet radiation;
two concentric chambers, respectively:
an outer chamber, the internal face of the wall of which exhibits a substrate covered with at least one photocatalysing agent;
an inner chamber, the wall of which is perforated, the external face of the said wall exhibiting a substrate covered with at least one photocatalysing agent;
inlet means for the gaseous effluent to be purified;
outlet means for the purified gaseous effluent;
the gaseous effluent to be purified being directed so as to lap at least the internal face of the wall of the outer chamber and to traverse the wall of the inner chamber.
In an advantageous embodiment, each of the outer and inner chambers is cylindrical in shape.
Furthermore, in order to initiate and to enhance the speed of the photocatalytic reaction, the source of UV radiation is provided in the form of at least one tubular UV lamp positioned between the wall of the cylindrical outer chamber and the wall of the cylindrical inner chamber, parallel to the said walls.
In order to allow the effluent to be treated to enter the plant, the inlet means for the gaseous effluent are provided in the form of at least one opening inserted in one of the faces of the cylindrical outer chamber.
The said opening is advantageously equipped with an injector.
In a preferred embodiment, the inlet means for the gaseous effluent are provided in the form of twelve openings provided with injectors distributed evenly over one of the faces of the cylindrical outer chamber, so as to obtain a uniform distribution of the gaseous effluent within the said cylindrical outer chamber.
In contrast, in order to allow the purified air to exit, the outlet means for the gaseous effluent are provided in the form of at least one opening inserted in the opposing face of the cylindrical inner chamber.
In order to improve the convection of the gaseous effluent to be treated, at least one of the two cylindrical chambers is driven with a rotary motion. In that way, the effluent to be treated moves tangentially to the wall of the cylindrical outer chamber according to a turbulent and thus lapping movement, being able thus simultaneously to lap the wall of the cylindrical outer chamber and that of the cylindrical inner chamber and then, finally, to traverse the wall of the cylindrical inner chamber.
In a first embodiment, each of the two cylindrical chambers rotates in the same direction.
In a second embodiment, each of the two cylindrical chambers rotates in opposite directions.
In a third embodiment, a single cylindrical chamber rotates.
In order to accelerate the departure of the treated effluent, the plant can also be equipped with suction means. Any known suction means can be envisaged.